What’s With the Mobs of Robins?

by GEAS Guest Blogger Terry Rich

I’m guessing no bird in our country is more readily recognized or more familiar than the American robin. It’s a really good name. Yes, blue jays and cardinals are also well-known yard birds to millions, but we don’t have them Out West. Other species are common and conspicuous in certain regions – I think of northern mockingbirds in the south. But no species can match the status of the American robin. Write me if you have another opinion on this!

We are lucky this winter in southwestern Idaho because we are inundated with this beautiful and pugnacious thrush. How many do we have this year? Good question. How about “lots.”

Stay Still, Will You Please

Robins are hard to count accurately because they fly back and forth in flocks of various sizes, as if being shuffled by some cosmic bird card dealer. For those of us who try hard to count birds for our eBird submissions, they are probably the toughest. Unlike some species, for example, cedar waxwings, they don’t move in tight little flocks where you at least have a chance of counting them. Or take Canada geese. Although they often move in large, scattered groups, they are big enough and slow enough you can often get a pretty good count, if you think fast.

But robins are a different deal. When birding with friends and encountering a flock devouring some kind of fruit on a dozen different trees, I find myself often turning to them and saying, “how many do you think?” The answer this winter has often been, 200 or 300. If my friend’s estimate is within 20-30 of mine, I call it good. If we’re farther apart, I go with my count. Hey, I have to sign the papers.

 I’ve received a number of queries this winter about Turdus migratorius. Understandably, many people think populations have exploded. Others wonder if they are returning early from migration. Yet others, like me, just wonder, what the heck?

Hey, I Hear There’s a Party

The most germane thing to know about our robins is they are very social in winter, as opposed to their one-lawn-per-bird rule during the breeding season, and they eat fruit at this time of year. Earthworms will do the rest of the year, but they aren’t very tasty when cold. Plus, the ground is frozen. So, flocks of robins locate fruit resources and then usually, but not always, hang around or return periodically until the fruit is gone.

We’ve had plenty of winters that are quite the opposite of this one. In some years, robins are hard to come by. To me, all that means is that they are somewhere else. But you don’t have to take my word for it. There are data.

According to the Breeding Bird Survey, robin populations in Idaho have been stable since 1966. Across the entire U.S., they have increased slightly (and statistically significantly) over that time. This is solid evidence that the population is not bouncing around as our winter numbers might otherwise suggest.

Another interesting and accurate picture of what we’re seeing is shown by data from the Christmas Bird Count. The graph of robins counted in winter bounces from high to low over the years like a game of Pong, turned on end. It’s really nuts! One winter we have none, and the next they’re everywhere. Then, back to none.

Is That An Alaska Plate?

We know generally what’s going on, but it’s not easy to figure out exactly what’s going on. American robins occur over almost all of North America and well into Mexico during the breeding season. In winter, the northern-most populations migrate south, but we have “resident” birds across the entire lower 48.

I put “resident” in quotes because, although we have robins in Idaho in summer and winter (to some degree), we don’t know if those robins hopping around in the front yard in July are the same birds stripping our juniper berries in January. There’s a reasonable chance our winter birds are coming from Canada or even Alaska, and our summer birds have moved elsewhere. Or maybe not. We don’t know.

This winter would give some aspiring ornithologist the chance to mark maybe thousands of robins in the Treasure Valley, and then try to relocate them this summer. Are they here? Are they up in the mountains? Are they in Alaska? These questions are easy to ask but not so easy to answer. If they are local birds, we’d figure it out pretty fast. A robin with colored leg bands sitting on the deck would be easy to see. Otherwise, good luck.

It's great fun to watch a flock descend on a fruiting tree or shrub. Birds in our yard come for mountain ash and juniper. In the neighborhood, they additionally choose hawthorn and crab apples. And right now, they are pounding rose hips in the foothills. Rose hips are relatively large and mostly composed of seeds. I’ve watched a number of robins try to swallow one of these seed bombs and then drop it. There is apparently enough difference in size to make some a bit too big, but plenty just right. These birds are getting their RDA of vitamin C, no problem.

American robin eating a rosehip by Terry Rich

But robins don’t just swallow all these seeds and walk (hop) away. Some seeds are regurgitated, and the rest are passed. Like many species of birds (and mammals), robins are excellent dispersers of seeds. All that feeding and flying around results in seeds all over the place.

Bird FUI - Flying Under the Influence

Robins are among a few species that can become intoxicated by eating naturally fermented fruit. I have not observed this myself, but there are solid reports of birds getting tipsy on bitter cherry (Prunus emarginata), blackberries, and various species of holly. They may have trouble standing, seem to fall asleep, or can’t fly.

One of their drinking buddies, so to speak, is the cedar waxwing. Waxwings seem to fare much worse, as there are many reports of this species getting hit by cars as they try to fly while under the influence. Don’t do it! Robins are more likely to just sit tight and wait for the effects to wear off. By the way, it is common to see a few waxwings hanging out in a big flock of robins.

Our robins will be heading back to where they came from before long. A few of them are starting to sing, which reveals that increasing day lengths are starting to have their effects. They’ll soon abandon their gregarious ways and start to think a lot more about finding one special robin out there who will share a nest with them.

It was great fun this winter. Thanks for the memories. See you next winter? Probably not.

You can reach Terry at terryrichbrd@gmail.com